Correlation between the Open-Circuit Voltage and Charge Transfer State Energy in Organic Photovoltaic Cells

In order to further improve the performance of organic photovoltaic cells (OPVs), it is essential to better understand the factors that limit the open-circuit voltage (V-oc). Previous work has sought to correlate the value of V-oc in donor acceptor (D-A) OPVs to the interface energy level offset (E-DA). In this work, measurements of electroluminescence are used to extract the charge transfer (CT) state energy for multiple small molecule D-A pairings. The CT state as measured from electroluminescence is found to show better correlation to the maximum V-oc than EDA. The difference between E-DA and the CT state energy is attributed to the Coulombic binding energy of the CT state. This correlation is demonstrated explicitly by inserting an insulating spacer layer between the donor and acceptor materials, reducing the binding energy of the CT state and increasing the measured V-oc. These results demonstrate a direct correlation between maximum V-oc and CT state energy.